Photoreceptors or ommatidia in the eye of a Horseshoe Crab (Limulus polyphemus). Much of the knowledge about how a human eye functions is a result of research done on the compound eye of the horseshoe crab. SEM X1000

The length of time the Limilus eye can be kept alive in the lab, the eyes simple construction and the easily accessible optic nerve made it a prime candidate for research.